1. Field of the Invention
The present invention generally relates to internal combustion engines for land vehicles and, more particularly, to internal combustion engines for powering snowmobiles.
2. Description of Related Art
Snowmobiles are popular land vehicles that are operated primarily in the winter over snowy terrain. A typical snowmobile comprises a frame assembly, a drive assembly that is coupled to the frame assembly and engages a ground surface to propel the snowmobile, an internal combustion engine supported by the frame for powering the snowmobile, and a transmission for transmitting power from the engine to the drive assembly.
In the past, two-stroke engines were typically used to power snowmobiles. More recently, however, four-cycle engines have been used, primarily for their reduced emissions. The four-stroke snowmobile engine typically comprises a crankcase assembly, a crankshaft supported for rotation within the crankcase assembly, a cylinder block assembly that extends from the crankcase, and a cylinder head assembly connected to an end of the cylinder block opposite the crankcase. A crankshaft is typically supported for rotation within the cylinder head to actuate a valve mechanism of the engine.
The crankshaft typically includes a drive sprocket located at one end of the crankshaft. The crankshaft may include a driven sprocket. The camshaft is driven from the crankshaft by a timing chain or belt that extends around the drive sprocket and the driven sprockets.
The transmission of the snowmobile is typically driven from the end of the crankshaft at which the drive sprocket is located. This end of the crankshaft typically has a relatively large diameter to transmit torque from the engine to the transmission. Because the drive sprocket is located at this same end of the crankshaft, the diameter of the drive sprocket is also typically relatively large.
For proper actuation of the valve mechanism, the driven sprockets of the camshafts generally must have a diameter that is twice the diameter of the drive sprocket. As a result, the size of the cylinder head which typically contains camshaft and driven sprockets must be relatively large. This undesirably increases the overall size of the engine.
In addition, in order to minimize engine vibrations during operation of the snowmobile, it is preferable that the center of gravity of the engine be located in the proximity of the cylinder (e.g., at the central axis of the cylinder in a single cylinder engine, or at the center of the group of cylinders in a multi-cylinder engine). However, because the large diameter end of the crankshaft extends from one side of the engine to power the transmission and the camshafts, the center of gravity of the engine typically is offset towards the end of the crankshaft.
A need therefore exists for a snowmobile having an improved four-cycle engine.